Fuel cells are characterised by their capacity of directly converting the chemical energy of a fuel, for example pure hydrogen or hydrogen-containing gas, to electrical energy with no intermediate combustion stage. This allows fuel cells to get rid of the constraints of Carnot's principle and therefore to have an intrinsically higher energy efficiency than the conventional generators.
Several types of fuel cells are known, among which membrane fuel cells are a solid state device with a particularly simplified internal structure and with a remarkable capacity both of providing the nominal power in a very limited time starting from zero output conditions, and of promptly responding to instant electrical energy requests. This set of characteristics makes membrane fuel cells very attractive for use in the automotive and stationary field for small power appliances, as is the case, very interesting under a commercial standpoint, of systems directed to private housing installations, hotels, telecommunication relays, as well as computing centres and hospitals as an emergency unit.
Alongside these features, membrane fuel cells present however also a few drawbacks: among these, particular relevance has the need of maintaining the proton conductive polymeric membrane in a fully hydrated state, being its conductivity precisely a function of its water content.
The membrane must be inert against the strongly aggressive action of peroxide and radical compounds which are formed as intermediate reaction products and for this reason the currently commercially available types consist of perfluorinated polymers. On the polymer chains are inserted sulphonic groups (—SO3H), which must be dissociated: the resulting free electric charge in fact determines a particular spatial orientation of the polymer chains with formation of reticular channels along which the ionic migration occurs. The dissociation, which is thus a mandatory passage for channel formation, only takes place when the membrane contains a certain water amount, that is when the membrane is characterised by a suitable degree of hydration. The membrane water content is the result of a delicate equilibrium between water formed during operation and water withdrawn from the gases flowing across the fuel cell. Water extraction can become dangerously high when the fuel cell is operated at moderate pressures as required to minimise the parasite energy consumption, which negatively affect the overall system efficiency. With moderate operative pressures, in particular close to atmospheric, the volumetric gas flow-rates result to be high: on the air side, the situation is then particularly critical, since to maintain a sufficient oxygen partial pressure also in the cell regions close to the outlet, the air is supplied in a substantially higher amount, typically double, than the theoretically required value. In order to decrease the water extraction so as to preserve the necessary membrane hydration, several devices are disclosed in the prior art directed to saturate the feed gases, and especially air, with water vapour at temperatures close to that of fuel cell operation. Air saturation is achievable in the simplest of manners by bubbling the air in suitable external saturators consisting of vessels where demineralised water is maintained at the desired temperature by thermal exchange, for instance with the cooling water of the fuel cell: nevertheless, having to maintain the thermal exchange surfaces within reasonable limits, the saturator average temperature results lower than that of the cell and hence air still has a potential capability of dehydrating the membrane, which is displayed in particular at high output conditions. A modification of this device, disclosed in patent application DE 103 04 657, provides that the liquid water dragged by the exhaust air and discharged from the fuel cell be separated and collected in a vessel integrated with the cell itself: in this way it is achieved both a simplification of the overall system since the external saturator is eliminated, and a higher thermal level for the water evaporation because of the improved thermal exchange. Also with this solution, however, the temperature of saturation, although increased, remains lower than that of fuel cell operation. To complete the air feed saturation with water vapour it would be then necessary to resort to additional sources of thermal energy with a consequent decrease of the overall system energy efficiency: a procedure of this kind is claimed in U.S. Pat. No. 6,350,535, wherein atomised liquid water is added to the air feed and the mixture so obtained is directed across a heat exchanger provided with the required thermal energy to evaporate the water. These devices in any case require level control instrumentation, pumps for water feeding, purge flow-rate control to prevent the build-up of impurities inevitably present, albeit as traces, in the water to evaporate, with sensible consequences on the total costs. In U.S. Pat. No. 6,066,408 a method of humidification is described comprising humidifying cells intercalated to the fuel cells of a stack: in this way the humidifying cells practically operate as cooling cells wherein the cooling is ensured by the evaporation of the water required to saturate the air which is made flow across them. The humidification temperature results higher than that obtained with the above discussed external saturators, but always lower than that of the fuel cells since some temperature difference is still required to maintain an adequate heat exchange rate. However, such a device is hardly efficient in the start-up phase and at low power output when the cell temperature is significantly lower than at regime operation.
In US 2001/10015501 the use of a apparatus commonly defined as enthalpic unit is disclosed. Such unit consists of a vessel divided into two compartments by a selective water-permeable membrane: the two compartments are respectively fed with air at ambient temperature to be directed to the stack and with water vapour-saturated warm exhaust air exiting the cells. A heat and water exchange takes place across the membrane from the exhaust to the air feed, which warms up and is humidified: also in this case, however, the final temperature of the air feed is certainly lower than the operating temperature of the fuel cells. A similar device is disclosed in DE 199 18 849 wherein the water and heat transfer does not take place across a selective membrane, but rather through the use of a rotating drum subdivided into sectors whose internal walls are provided with a film of hygroscopic material, for instance a lithium salt. The rotation of the drum puts each sector subsequently in communication first with the exhaust air which transfers its water content to the hygroscopic material, then with the dry air feed which warms up and extracts water from the hygroscopic material. Of course, this device is as well subject to the previously mentioned limitations.
In U.S. Pat. No. 5,441,821 a certain moisture and thermal level is achieved through the recycle of exhaust air on the air feed fan or compressor: in this case, supposing that the exhaust air is saturated with water vapour, the resulting humidity of the overall air stream is a function of the ratio between the flow-rates of the recycle and of the fresh air from the environment. Since this ratio cannot be very large in order to contain the size of fans or compressors and the relevant energy consumption within reasonable limits, the overall air humidity content is again unsatisfactory. Furthermore the recycle of oxygen-depleted exhaust air implies the average partial pressure of oxygen inside the fuel cells to be lower than that characterising the operation without recycle. This may imply a certain lessening of the performances.
To obviate to the above inconveniences while achieving a safe and complete membrane hydration, U.S. Pat. No. 6,406,807 discloses the direct injection of water inside the fuel cells: the evaporation effectively withdraws the reaction heat simultaneously generating the vapour partial pressure required for maintaining a correct hydration of the membranes. The method is somehow critical in consideration of the fact that the amount of water has to be calibrated as a function of power output to prevent the two opposed hazards of hydration loss (injection of an insufficient amount of water) and of electrode flooding (injection of an excessive amount of water).
A further way of facing the problem of membrane dehydration is discussed in US 2002/0068214: besides the humidification carried out by means of one of the above disclosed processes, the membrane fuel cell cathode has a reduced porosity in the air inlet region, which is more exposed to the risk of excessive water evaporation. In this way the water withdrawal as vapour or even as liquid results to be the more hindered the lower is the residual porosity, with a better preservation of the membrane hydration. This procedure entails two serious inconveniences, one associated with the simultaneous decrease of oxygen diffusion rate leading to performance drop, and the other with the more complex electrode structure, badly fitting the requirements of large scale and low cost production.
The above types or air feed conditioning, due to their intrinsic limitations, are acceptable only in case of high pressure operation, typically from 3 to 4 bar, that is when the air feed has a substantially reduced volumetric flow-rate and may have a temperature higher than ambient under the effect of compression, while they are arguable and poorly reliable for operation at pressure below 3 bar, in particular below 2.5 bar.
The basic aim of the present invention is to overcome the limitations of the prior art by disclosing a fuel cell which can be operated with non-externally humidified air also at moderate pressures.